This invention relates to a process to produce resin matrix materials. More particularly, it relates to an improved method to prepare resin-reinforced fiber composites, such as sheets and tapes, and their consolidation under heat and pressure into laminates.
Composites of resins and fibrous reinforcements are employed in airfoils such as spoilers, wings, flaps, ailerons, rudders, vertical stabilizers, horizontal stabilizers, helicopter blades, as well as other structural components requiring high stiffness and light weight. For example, the fibrous reinforcement can comprise glass fibers, graphite fibers and high temperature resistant fibers, such as polyaramid fibers and these can be unidirectional, as in tapes, random as in mats and felts, or woven, knitted, and the like. State of the art airfoils generally are made of graphite fiber resin composites in a plurality of plies, e.g., 5, consolidated under heat and pressure into a laminate in the shape of the airfoil being designed.
The resins which can be employed in the manufacture of such composites are generally curable thermosetting resins and/or thermoplastic resins which are high temperature resistant. Curable epoxy resins can be used by applying them to the fibers by any suitable method such as spraying or impregnation, for example, during winding. Solvents for the resins are often employed depending on the properties of the resin to insure thorough wetting of the fibers with the resin. Such resins are partially cured or B-staged to solidify the resin and to provide a fiber-reinforced composite ply which is self-supporting, a so-called pre-preg. Prepregs in the form of tapes, sheets and the like are then employed to prepare laminates, such as airfoils, by forming into an assembly and molding under heat and pressure by common techniques. See, for example, Jensen, U.S. Pat. No. 3,768,760.
Exemplary resins which can be employed include epoxy novolacs, polyimides, and other epoxies of two well known types, e.g., the bis-phenol epichlorohydrin and the bis(epoxy-cyclopentyl)ether types. Solvents which can be employed with such resins to aid in wetting the fibers, especially graphite fibers, are methyl ethyl ketone, acetone, ethanol, and mixtures thereof.
A more recent development is to use as the resin components particular families of polyimides, such as the polyimides derived from benzophenone tetracarboxylic anhydride and diaminoarenes, see, e.g., Alberino, et al., U.S. Pat. No. 3,930,097, and especially the thermoplastic polyetherimides derived from an aromatic bis-(ether anhydride) and an organic diamine, e.g., those of Takekoshi, et al., U.S. Pat. No. 3,917,643. These are difficult to dissolve in common solvents and, accordingly, they are usually applied to the fiber by either melt impregnation, which is difficult from a manufacturing standpoint and leads to poor fiber wetting, or by solvent impregnation from methylene chloride which gives poor fiber wetting, low resin uptake, and leads to distortion of the prepreg due to rapid solvent evaporation. In White, U.S. Pat. No. 4,049,613 are disclosed carbon fiber - polyetherimide matrix composites, in which the polyetherimide has terminal nitro groups. The resin is deposited on the carbon fiber from "a suitable organic solution in chloroform". Apart from the fact that an unconventional nitro-terminated polyimide must be used, the well-known adverse physiological effects of chloroform, especially 100% chloroform, must be considered disadvantageous.
It has now been discovered that, if a judicious selection of solvents is made, in formulating a multicomponent solvent mixture, that thermoplastic polyimides can be formed into prepregs by deposition from solutions using entirely conventional manufacturing equipment. The need to use expensive dipolar aprotic solvents, such as dimethyl formamide, dimethyl acetamide, dimethyl sulfoxide, dimethyl sulfone, hexamethylphosphoramide, N-methyl-2-pyrrolidone, tetramethylurea, pyridine, and the like, as taught by above-mentioned U.S. Pat. No. 3,930,097, or toxic and caustic phenol, as taught by above mentioned U.S. Pat. No. 3,917,643, and toxic 100% chloroform as taught by U.S. Pat. No. 4,049,613, is avoided. A very high quality prepreg is obtained, with high resin uptake and no distortion.